Wonder why they adopted the terminology "part time" and "full time"....
Okay, sit back and grab a cup of coffee, folks, it's time for a 4wd history lesson.
Pay attention, as there will be a quiz afterwards.
It goes back to the old days of 4wd. 4wd was virtually unheard of in civilian vehicles until after WWII. The production of the Jeep and the Dodge WC series of 4wd trucks during the war made 4wd a more popular option.
Still, Ford and Chevy didn't offer factory 4wd on their trucks until the late 1950s (if you wanted a 4wd before then, you would arrange to purchase a 2wd truck from the dealer and the dealer would then send it to an aftermarket shop - NAPCO or Marmon-Herrington - who would then do the 4wd conversion and send it back to the dealer.) Jeep and Dodge were a little bit ahead of the game with their 4x4 trucks, both introduced in the late 1940's (the first "wagon" type 4wd SUV was the 1949 Jeep Utility Wagon.)
Anyway, back then there were no center differentials and everybody who bought a 4x4 understood that you could not use the 4wd on dry, hard-surfaced roads because of the potential for driveline binding.
That situation existed until the early 70's when the first commercially viable full-time 4wd vehicle, the Range Rover, was introduced. The Range Rover used a revolutionary Inter-Axle Differential (what we would call the center differential) to prevent driveline binding and allow the vehicle to be in 4wd all the time. Shortly thereafter, around 1974-75, all of the Domestic manufacturers started introducing full-time 4wd vehicles, most of which used the New Process 203 transfer case with an inter-axle differential.
On Ford, Chevy and Dodge vehicles, the IAD (Inter-Axle Differential) was an open diff and there was a lever on the floor that allowed 5 modes: Low-Lock (low range, diff locked), Low (low range, diff open), Neutral, High (high range, diff open) and High-Lock (high range, diff locked.) They were sold as "full-time four wheel drive" because they could be used "full time", that is, there was no 2wd option, and it was contrasted with the conventional 4wd systems which were "part time" because you could not use 4wd all the time (unless you did
all your driving on unimproved or slippery roads.)
Jeep used a slightly different version of the NP203 in their Wagoneer and J-series trucks. On those vehicles, the center diff was a limited slip. Because of this, it was thought that under normal circumstances, there would be no need to lock it, so Jeep "hid" the lock button in the back of the glove box. On some NP203 equipped vehicles like the Wagoneer, the transfer case was a single-speed unit and low range was in an optional module that bolted onto the back of the T-case, and there was a small lever on the floor for engaging low range. AFAIK these t-cases did not have a neutral position.
Full time 4wd was hailed as a great acheivement when it came out. Magazines pointed out that the primary advantage of full time 4wd was not
off-road - where the t-case would normally be locked anyway - it was
on-road where the use of 4wd on pavement could allow for better handling and better response to sudden patches of rain, ice, snow, etc.
Although all three manufacturers had full time 4wd on their vehicles, they differed on how they offered it. Chevy/GMC and Jeep generally put full time together with automatic transmissions. IOW, if you bought a vehicle with an AT, you got full time 4wd, and if you bought one with a manual tranny, you got part time. With Ford, I believe it was an option with the automatic and not available with the manual. Dodge, OTOH, was fully committed to the full-time system and made it standard across their line.
Incidentally, International Harvester, which was also a player in the recreational 4x4 field with the Scout, Scout II, Travelall, Traveler, Terra and the various pickups they built,
never offered full time 4wd on any of their vehicles.
Unfortunately, the year full time 4wd was introduced - 1974 - was also the year of the first Arab oil embargo (a response to the US support of Israel during the Yom Kippur war of 1973.) The oil embargo caused gas prices to spike all across the country and also led to intermittent shortages. This was a problem for the 4x4 manufacturers as full time 4wd vehicles used more gas than the older, part-time 4wd vehicles.
By the late 70's gas prices were still high and some aftermarket manufacturers were coming up with ways of temporarily deactivating the full time T-case. By the early 80's most of the big 3 had abandoned full time 4wd because it hurt their Corporate Average Fuel Economy (CAFE) requirements.
With a few exceptions (like some of the Grand Wagoneers introduced in the early 80's) this was the situation that existed for the next 10 - 15 years.
(to be continued...)
